The present invention relates to an image sensor which is employed for reading out, for instance, printed material for facsimile transmission, and converts an image into a corresponding electric signal.
Of image sensors heretofore used in facsimile, there is an image sensor that is formed by a silicon chip of a size about 20 to 30 mm.sup.2 and having incorporated therein 2000 or so photo diodes and a scanning circuit therefor through utilization of MOS (Metal-Oxide-Semiconductor), CCD (Charge Coupled Device) or like semiconductor integrated circuit technology. In order to read out, for example, a printed page by this image sensor, it is necessary to form a reduced image on the sensor through using a lens and, to perform this, the optical path length from the printed page to the sensor, resulting in the equipment becoming bulky as a whole. For example, in the case of an A4-size printed page (200 mm wide) the reduction ratio for forming reduced images on the sensor using a lens with a focal length f of 50 mm is about 1/10 and the optical path length exceeds 500 mm. Further, this image sensor involves complex adjustment for image formation and employs a fluorescent lamp as a light source, but its lifetime is short.
As a solution to such defects, there have been proposed large image sensors of the same size as the width of the printed page. According to one of such large image sensors, a number of optoelectro transducers are formed in a line through the thin film technology and the length of the optoelectro transducer array is made identical with the width of the printed page. The printed page is held substantially in close contact with the sensor, permitting the readout of the printed page without using a reducing lens. Some of such image sensors employ a drive circuit exclusively for each optoelectro transducer as scanning circuits for sequentially taking out the outputs from the optoelectro transducers. In this case, for instance, when the number of optoelectro transducers used is 1728, the number of connection points of external integrated circuits for the scanning circuit and the optoelectro transducers is large and the number of integrated circuits for the scanning circuits is also large; accordingly, this method is economically disadvantageous.
In such an image sensor which reads out written, typed, printed or photographic material held in close contact therewith, it has been proposed, with a view to reducing the number of connection points of the integrated circuits as scanning circuits for taking out the optoelectro transducer outputs and the optoelectro transducers, to adopt what is called matrix wiring and connect the optoelectro transducer at the intersections of row and column lines so that the transducers are selectively energized by selecting the row and column lines one by one. In the image sensor utilizing the matrix wiring, however, the so-called cross talk occurs, extremely lowering the SN ratio. To avoid this, it is customary in the prior art to connect a blocking diode in series to each optoelectro transducer and cut off the crosstalk through utilization of the backward characteristic of the blocking diode as set forth, for example, in U.S. Pat. No. 3,544,713 ("Solid State Electro-Optical Contact Scanner" issued on Dec. 1, 1970). With this method, however, a special manufacturing step is required for forming the blocking diodes on the same substrate together with the optoelectro transducers and, in addition, it is difficult to make the forward contact resistances and the backward resistances of the blocking diodes uniform, which leads to dispersion in the transducer outputs. Accordingly, this image sensor necessitates the use of a compensation circuit for compensating for such dispersion, and hence is uneconomical. Besides, since the optoelectro transducers cannot be selected and driven at high speed owing to the presence of the blocking diodes, the image sensor can be used only with low-speed facsimile.